Sacramento County coroner's spokesman Ed Smith told the Sacramento Bee newspaper that the infant's fourth-degree burns were among the worst investigators there had seen.

"The child was apparently killed by the burning of the tissue," Smith told the Bee. "I don't know if they can say how long baby was in the microwave."

Peck says police do not have a motive. No attorney was listed for Yang.

Police said Yang has three other young children, all boys ages 7 and younger. They have been removed from her home.

Last month, an Ohio woman convicted of killing her month-old baby daughter in a microwave oven was spared the death penalty and sentenced to life in prison without parole.

China Arnold, 31, of Dayton, was convicted of aggravated murder. Prosecutors say Arnold intentionally put 28-day-old Paris Talley in a microwave and turned it on after a fight with her boyfriend. The couple had argued over whether the boyfriend was the infant's biological father.